1. Technical Field
The invention relates to a connecting rod/bearing combination having a bearing (1) for a small connecting rod eye (6) of a connecting rod (5), which serves to accommodate a piston pin mounting a piston and a method of producing such a connecting rod/bearing combination.
2. Related Art
The background of the invention is the problem of the mass forces arising in a driving mechanism. In the context of the present invention, a driving mechanism should be understood to mean the system consisting of piston, connecting rod and crankshaft, as used for example as a crank mechanism in internal combustion engines.
Irregular movements of masses, i.e. accelerated and decelerated movements, involve mass or inertial forces. The piston of an internal combustion engine moves substantially in a straight line in the direction of the axis of the cylinder liner and effects an oscillating movement. In the process, it is exposed to constantly varying accelerations, which reach their maximum at the reversal points of the piston movement, the top and bottom dead centers. Corresponding to the rectilinear movement of the piston, the accelerations and with them the inertial forces are directed in the direction of the cylinder liner axis.
In contrast to the piston, the crank effects not an oscillating but a rotating movement. The inertial forces acting thereon are the centrifugal forces produced during rotary motion by the constant change in direction.
The most complex movement of the three components forming the crank mechanism is performed by the connecting rod. Movement of the connecting rod is a composite movement and comprises components of both oscillating and rotary motion.
FIG. 1 is a schematic representation of a driving mechanism (20) having a piston (21), a connecting rod (22) and a crank (23). A counterweight (26) is shown in dash-dotted lines.
FIG. 2 shows an equivalent system for the driving mechanism (20) shown in FIG. 1, wherein the equivalent system is composed of two point masses (24, 25), of which one sub-mass mh (24) is arranged in the piston pin center and performs oscillating motion and one sub-mass mr (25) is arranged in the center of the cranked portion (23) of the shaft and effects rotary motion with the crankshaft.
The background to this equivalent system is that the connecting rod mass mP may be broken down relative to the mass action of the connecting rod into an oscillating mass mph and a rotating mass mPr. These equivalent masses are conventionally determined according to the following equations:mP=mPr+mPhmPr×11=mPh×12,wherein 11, 12 indicate the spacing of the center of gravity of the connecting rod from the equivalent point masses.
The oscillating equivalent mass mh (24) is obtained from the sum of the piston mass mK and the oscillating connecting rod mass mPh. Accordingly, the rotating equivalent mass mr (25) is obtained from the sum of the rotating connecting rod equivalent mass mPr and the shaft cranked portion mass mKur reduced to the crank radius r, which is calculated as:mKur=mKu r1/r
Here, r1 is the spacing of the center of gravity of the cranked portion (23) of the shaft from the axis of rotation of the crankshaft, mKu is the mass of the cranked portion of the shaft and r is the crank radius.
The mass forces, which are produced by the rotating mass mr (25), may be simply counterbalanced by an opposing counterweight (26). On the other hand, minimization or compensation of the inertial forces produced by the oscillating mass mh (24) is problematic.
In general, research and development work is aimed at reducing the masses of all oscillatingly moved parts, in particular the piston, since compensation of the mass forces produced thereby cannot be handled by the arrangement of counterweights. On the one hand, redesign of the reciprocating components using the smallest possible amount of material is necessary, while on the other hand the use of new materials of lower relative density will also achieve the objective.
However, this procedure is also subject to limitations due to strength requirements, such that it becomes ever more difficult to minimize the oscillating masses and the mass inertial forces caused thereby.
In the case of multi-cylinder engines, it is possible to influence the mass forces or the effect thereof by the number of cylinders, the cylinder arrangement, the crank sequence of the crankshaft and/or the ignition sequence.
If, for example, in the case of a multi-cylinder engine, it is intended to compensate the mass forces by means of a cylinder arrangement selected for this reason, this ideally requires the same driving mechanisms for all the cylinders, comprising the same weight or the same weight distribution. It is thus ensured that the driving mechanisms used in one and the same engine produce uniformly large mass forces, especially uniformly large oscillating or rotary mass forces.
For manufacturing reasons, the components used do not exhibit exactly the desired weight or the desired weight distribution, and thus nor do the driving mechanisms composed thereof.
The prior art has attempted to compensate the deviation, caused by manufacture, of the individual components with regard to mass and mass distribution as follows:                connecting rod and piston are subjected in each case individually to a selection process, in which they are divided into a large number of weight categories, and        connecting rod and piston are then grouped, with the objective of achieving similar connecting rod/piston combinations,        wherein the connecting rod comprises counterweights both at the connecting rod eye and at the connecting rod head, which are machined to achieve exact adjustment of the mass or mass distribution of the connecting rod/piston combinations.        
In the process, it is necessary to take into account the fact that the counterweights which are provided at the connecting rod eye and at the connecting rod head and are also known as cams increase the total weight of the connecting rod and thus the total weight of the driving mechanism. This means that merely providing these counterweights for the purpose of adjusting the mass or mass distribution increases the weight and is thus inconsistent with the real objective of minimizing the masses of all the moving components.
For this reason, the attempt has been made to provide high-precision manufacturing methods with which low tolerances may be achieved. By reducing or decreasing the manufacturing tolerances, the differences between the components of a particular type are reduced at the same time, such that the size of the counterweights to be provided may likewise be reduced.
Production of the connecting rod with lower mass or mass distribution tolerances has been made possible by more recent methods, such that the number of weight categories necessary for selection has been greatly reduced and, in the case of maximally optimized processes, only connecting rods of one weight class remain.
When grouping these connecting rods with the associated pistons to form a connecting rod/piston combination, compensation of the connecting rod mass tolerances is effected by the piston. Nevertheless, there is a requirement for precision fine-tuning of the oscillating masses.